Electroluminescent materials

ABSTRACT

Highly luminous, thermally stable and moisture-resistant light-emitting materials derived from quadridentate ONNO-type ligands and a Group 10 metal were employed as emissive dopants in organic light-emitting devices. The dopants have molecular structures represented by the formula I and II:  
                 
 
     wherein M represents Group 10 metal (including platinum) and R 1 -R 14  are each independently selected from the group consisting of hydrogen; halogen; alkyl; substituted alkyl; aryl; substituted aryl, with substitutents selected from the group consisting of halogen, lower alkyl and recognized donor and acceptor groups.

FIELD OF THE INVENTION

[0001] The present invention relates to light-emitting materials, whichcan be deposited as a thin layer by vacuum deposition, and which can beused as effective dopants in organic light-emitting devices (OLEDs).

BACKGROUND OF THE INVENTION

[0002] The progress of light-emitting diode (LED) over the past twodecades has primarily focused on inorganic types because earlydevelopment in organic light-emitting devices (OLEDs) resulted in poorfabrication and packaging, and short lifetimes. Today, galliumarsenide-based LEDs in the market are commonly available withefficiencies in some spectral regions exceeding conventional filteredfluorescent lamps. However, in the development of light-emittingmaterials for display technology, inorganic semi-conductor materials arenot compatible for large-area assembled displays.

[0003] Pope et al. at New York University demonstrated organicelectroluminescence in the 1960s based on anthracene materials (J. Chem.Phys. 38, 2042, (1963)). Much progress have been made since thediscovery of the tris(8-hydroxyquinolato)aluminum (Alq₃) based thin filmdevice by C. W. Tang et al. at Kodak (Appl. Phys. Lett. 51, 913,(1987)). These contributed largely to the continuous discovery of newand improved electroluminescent materials. From small fluorescentmolecules to conjugated polymers, many OLEDs have been shown to exhibitsufficient brightness, remarkable efficiencies, good operating lifetimesand desirable ranges of color emission.

[0004] Organic light-emitting devices containing metal complexes are ofparticular interest because of their unusual chemical and electronicproperties. Some compounds bearing heavy metals exhibit potentialadvantages for OLEDs owing to their high internal quantum efficiencies.Conventionally, fluorescent materials are employed as dopants inemissive hosts. Singlet excitons (maximum theoretical internal quantumefficiency=25%) are formed after recombination of hole and electron toemit electroluminescence via dipole-dipole interaction through Forstermechanism (U.S. Pat. No. 6,310,360). Whereas, for heavy metal complexes,strong spin-orbit coupling can lead to singlet-triplet state mixing,which can result in high-efficiency electrophosphorescence in OLEDs(theoretical internal quantum efficiency up to 100%) (Nature, 395, 151,(1998); Synthetic Metals, 93, 245, (1998); Appl. Phys. Lett. 77, 904,(2000)).

[0005] However, some phosphorescent materials have intrinsicdisadvantages, such as saturation of emission sites due to excessivelylong lifetimes as well as triplet-triplet annihilation and concentrationquenching arising from strong intermolecular interactions at high dopinglevels (Phys. Rev. B. 60, 14422, (1999)).

[0006] For example, quadridentate azomethine-zinc complexes have beenused as blue light emitters in organic light-emitting devices, whichexhibit maximum luminance of approximately 1000 cd/m² only (Jpn. J.Appl. Phys., 32, L511 (1993); U.S. Pat. No. 5,432,014).

[0007] Azomethine-aluminum/gallium complexes have been employed in OLEDsas emissive materials. The current density of the device containingazomethine-gallium complex is 1 mA/cm² at 10 V and theelectroluminescence is greenish blue (U.S. Pat. No. 6,316,130).

[0008] It is therefore desirable to develop emissive dopant materialsthat can permit efficient energy transfer between the host and dopant inOLEDs, while causing little or no self-quenching even at sufficientlyhigh doping concentrations.

SUMMARY OF THE INVENTION

[0009] Examples of objects of the present invention in embodimentsthereof include:

[0010] The main objective of this invention is to prepare organiclight-emitting devices (OLEDs) doped with new light-emitting materials.The devices exhibit low turn-on voltages and high luminance andefficiencies.

[0011] An object of the present invention is to provide thermallystable, moisture-resistant metal-chelated materials that can bedeposited as a thin layer of known thickness by a vapor depositionprocess.

[0012] Further, the present invention concerns the design of highluminous dopants, which can be used at low concentration levels inlight-emitting devices.

[0013] New light-emitting materials derived from quadridentate ONNO-typeligands, and a Group 10 metal (including platinum) were prepared asillustrated by formula I and II:

[0014] wherein M represents Group 10 metal (including platinum) andR₁-R₁₄ are each independently selected from the group consisting ofhydrogen; halogen; alkyl; substituted alkyl; aryl; substituted aryl,with substitutents selected from the group consisting of halogen, loweralkyl and recognized donor and acceptor groups.

[0015] Embodiments of the present invention includes, but is not limitedto, OLEDs comprising heterostructures for producing electroluminescencewhich contain anode (ITO glass substance), hole transport layer (NPB(α-naphthylphenylbiphenyl amine)), matrix emissive layer [host material(beryllium bis(2-(2′-hydroxyphenyl)pyridine) (Bepp₂)) with differentconcentration of dopants as illustrated by formula I and II herein],charge transport layer (lithium fluoride) and cathode (aluminum metal).

[0016] The preferred embodiment as an effective dopant in the OLEDsherein is:

[0017] The present invention provides new materials for applications asemissive dopants in electroluminescent devices. The invention includesthe synthetic methods for these novel complexes plus their use aslight-emitting materials. The devices of the present invention can beapplied to field of display, light-emitter, display board for sign lamp,or light source for liquid crystal display.

BRIEF DESCRIPTION OF THE FIGURES

[0018]FIG. 1. Absorption spectra of complexes 1b & 2b in CH₂Cl₂

[0019]FIG. 2. Emission spectra of complex 1b in CH₂Cl₂ and as thin filmat 298 K

[0020]FIG. 3. Emission spectra of complex 2b in CH₂Cl₂ and as thin filmat 298 K

[0021]FIG. 4. TGA thermograms of complexes 1b and 2b under nitrogen andair

[0022]FIG. 5. Schematic diagram of OLED in present invention

[0023]FIG. 6. Electroluminescent spectrum, currentdensity-voltage-luminance curves of Device A containing complex 1b(doping level 0.3 wt %)

[0024]FIG. 7. Electroluminescent spectrum, currentdensity-voltage-luminance curves of Device B containing complex 1b(doping level 1.0 wt %)

[0025]FIG. 8. Electroluminescent spectrum of Device C containing complex1b (doping level 2.0 wt %)

DETAILED DESCRIPTION OF THE INVENTION

[0026] The inventions are generally related to syntheses, spectralcharacterization, phosphorescence, light-emitting properties of the newlight-emitting materials, and their applications in OLEDs. The examplesare set forth to aid in an understanding of the inventions but are notintended to, and should not be interpreted to, limit in any way theinvention as set forth in the claims which follow thereafter.

[0027] The examples given illustrate the synthetic methods of ligands laand 2a, and the platinum complexes 1b and 2b. The quadridentateONNO-type ligands la and 2a were prepared by modification of aliterature procedure (J. Chem. Soc., Perkin Trans. 2, 863, (1998)).Another example of the preparation of ONNO-type ligands has beenreported (U.S. Pat. No. 6,177,419).

EXAMPLE 1

[0028] Synthesis of 1a

[0029] A mixture of6,6′-bis(2-methoxyphenyl)-4,4′-bis(tert-butyl)-2,2′-bipyridine (1 g) inhydrobromic acid (47%, 20 mL) was refluxed for 12 hours. This was cooledto room temperature and was neutralized with an aqueous saturated Na₂CO₃solution at room temperature. The organic product was extracted withchloroform and the extracts were washed with deionized water (50 mL x2), dried over anhydrous Na₂SO₄, and a solid residue was obtained byremoval of solvent. Crystalline product of la was obtained byrecrystallization from a methanol/dichloromethane solution. EI-MS (m/z):452 [M]⁺. ¹H NMR (CDCl₃, δ, ppm): 14.45 (2H, s, OH), 8.16 (2H, d, J=1.4Hz, ArH), 7.97 (2H, d, J=1.3 Hz, ArH), 7.90 (2H, dd, J=8.0 Hz, J=1.4 Hz,ArH), 7.34 (2H, td, J=8.4 Hz, J=1.5 Hz, ArH), 7.07 (2H, dd, J=8.2 Hz,J=1.6 Hz, ArH), 6.96 (2H, td, J=8.1 Hz, J=1.2 Hz, ArH), 1.47 (18H, s,^(t)Bu). ¹³C NMR (CDCl₃, δ, ppm): 163.3, 159.7, 157.5, 152.2, 131.5,126.5, 119.2, 118.9, 118.4, 116.4, 35.6, 30.6.

EXAMPLE 2 Synthesis of 1b

[0030]

[0031] The synthetic method of metal complex 1b is described. A mixtureof NaOMe (0.014 g, 0.25 mmol) and the ligand la (0.113 g, 0.25 mmol) inmethanol (20 mL) was stirred for 2 hours. An acetonitrile solution (20mL) of Pt(CH₃CN)₂Cl₂ (0.25 mmol) was added to the methanolic suspension,which was refluxed for 24 hours. The resulting suspension was filteredand concentrated to about 5 mL. Upon addition of diethyl ether, abrown-yellow solid was obtained. The crude product was recrystallized bydiffusion of diethyl ether into a dichloromethane solution to affordyellow crystals. FAB-MS (m/z): 645 [M]⁺, 1292 (2M++2), 1938 (3M++3). ¹HNMR (CDCl₃, δ, ppm): 8.32 (d, 2H, J=1.41 Hz ArH), 8.01 (d, 2H, J=7.25Hz, ArH), 7.85 (d, 2H, J=1.68 Hz, ArH), 7.48 (dd, 2H, J=7.38 Hz, J=1.13Hz, ArH), 7.38 (td, 2H, J=5.35, 1.61 Hz, ArH), 6.79 (td, 2H, J=5.40,1.35 Hz, ArH), 1.54 (s, 18H, ^(t)Bu). ¹³C NMR (CDCl₃, δ, ppm): 162.745,159.105, 155.291, 149.851, 131.269, 128.005, 124.060, 120.465, 120.402,116.302, 116.148, 30.402, 29.715. FTIR (KBr, cm⁻¹): 3086 w, 2953 m, 1612w, 1528 s, 1351 s, 1034 m, 885 w, 723 m.

EXAMPLE 3

[0032] Synthesis of 2a

[0033] In a 100 mL round-bottom flask equipped with a reflux condenser,2,9-bis(2-methoxyphenyl)-4,7-diphenyl-1,10-phenanthroline (finelyground, 2 g, 3.7 mmol) and pyridinium hydrochloride (4.23 g, 37 mmol)were mixed. The mixture was heated under nitrogen flow to 210° C. for 36hours. After cooling, water (30 mL) was added and the aqueous solutionwas extracted with chloroform (3×30 mL). Combined organic extracts werewashed with saturated sodium bicarbonate solution (5×30 mL) and water(3×30 mL), dried over anhydrous magnesium sulfate and evaporated to givea bright yellow solid. Chromatography over silica gel using n-hexane:dichloromethane (1:2) as eluent afforded 0.99 g of a yellow solid.FAB-MS (m/z): 517 [M+H]⁺. ¹H NMR (300 MHz, CDCl₃, δ, ppm): 14.69 (2H, s,OH), 8.52 (2H, s, ArH), 8.41 (2H, dd, J=8.0, 1.3 Hz, ArH), 7.90 (2H, s,ArH), 7.71 (4H, d, J=7.4 Hz), 7.64 (6H, m, ArH), 7.43 (2H, td, J=7.7,1.5 Hz, ArH), 7.10 (2H, dd, J=7.4 Hz, 1.3 Hz, ArH), 7.04 (2H, td, J=7.5,1.3 Hz, ArH). ¹³C NMR (600 MHz, CDCl₃): δ=160.5, 157.7, 150.3, 142.8,137.8, 132.2, 129.6, 128.9, 128.8, 127.1, 125.7, 123.7, 120.6, 119.4,119.2, 118.9.

EXAMPLE 4

[0034] Synthesis of 2b

[0035] K₂PtCl₄ (0.08 g, 0.19 mmol) and 2a (0.1 g, 0.19 mmol) wererefluxed in glacial acetic acid (10 mL) for 2 days. After cooling, theresulting suspension was collected by filtration, washed with aceticacid and water successively and then dried under vacuum to afford abrown solid. The crude product was purified by chromatography on asilica gel column with dichloromethane as eluent. The product wasrecrystallized by slow evaporation of a dichloromethane solution toafford red crystals. FAB-MS: m/z=710 [M+H]⁺. ¹H NMR (270 MHz, DMSO-d₆)8.81 (2H, s, ArH), 8.56 (2H, d, J=8.9 Hz, ArH), 8.01 (2H, s, ArH), 7.86(4H, m, ArH), 7.71 (6H, dd, J=4.9, 2.0 Hz, ArH), 7.44 (2H, t, J 7.4 Hz,ArH), 7.24 (2H, d, J=8.2 Hz, ArH), 6.80 (2H, t, J=7.6 Hz, ArH).

[0036] The spectral characteristics of the platinum complexes 1b and 2baccording to this invention are shown in Table 1. For complex 1b, strongabsorption bands at 250-350 nm (ε=38400-17500 dm³ mol⁻¹cm⁻¹) and amoderately intense absorption band at λ_(max) 398 nm (ε=10800 dm³mol⁻¹cm⁻¹) are observed (FIG. 1). In addition, a broad absorption bandcan be found at ca. 480 nm (2800 dm³ mol⁻¹ cm⁻¹). For complex 2b (FIG.1), several vibronic transitions at 291-375 nm (ε=39200 to 24700 dm³mol⁻¹cm⁻¹) and a broad band at 504 nm (ε=7200 dm³ mol⁻¹cm⁻¹) areobserved in CH₂Cl₂. TABLE 1 UV/vis absorption data of 1b and 2b inCH₂Cl₂ Complexes λ_(max)/nm (ε/10⁴ × dm₃mol⁻¹cm⁻¹) 1b 255 (3.80), 315(1.75), 400 (0.82), 480 (0.25), 505 (0.22) 2b 291 (3.92), 315 (3.40),325 (3.23), 352 (2.58), 375 (2.47), 420 (0.52), 488 (0.67), 504 (0.72)

[0037] The photoluminescence (PL) of the platinum complexes 1b and 2b insolution and as thin film are summarized in Table 2. In FIG. 2, the 298K structureless emission of complex 1b are observed at 595 and 599 nm inCH₂Cl₂ and as thin film respectively. The PL properties of complex 2b insolution and as thin film are shown in FIG. 3. The emission maximum of2b in thin film is shifted by 1704 cm⁻¹ compared to that in solution.Meanwhile, complexes 1b and 2b exhibit lifetimes of 1.9 and 5.3 μs inCH₂Cl₂ and luminescent quantum yields of 0.1 and 0.6 (with Ru(bpy)₃Cl₂as reference standard) respectively. TABLE 2 PL properties of complexes1b and 2b in solution and as thin film Complexes (Measuring medium/Emission Temperature) (Maximum/nm) Lifetime (μs) Quantum yield 1b(CH₂Cl₂/298 K) 595 1.9 0.1 1b (Thin Film/298 K) 599 / / 2b (CH₂Cl₂/298K) 586 5.3 0.6 2b (Thin Film/298 K) 651 / /

[0038] The TGA thermograms of complexes 1b and 2b are shown in FIG. 4.Both the complexes demonstrate high thermal stabilities in nitrogen andair at heating rate of 15° C./min. Complex 2b is stable up to 536° C. innitrogen and 379° C. in air. The on-set temperatures of 1b are at 438°C. in nitrogen and 382° C. in air. These observations reveal that theselight-emitting materials can be sublimed and stable at vacuum depositionconditions in preparation of OLEDs.

[0039] An electroluminescent device according to this invention isschematically illustrated in FIG. 5. As examples of the presentinvention, OLEDs with configurations of ITO/NPB(α-naphthylphenylbiphenyl amine) (500 Å)/[0.3 wt % (device A), 1 wt %(device B), or 2 wt % (device C) of complex 1b]:Bepp₂ (400 Å)/LiF (15Å)/A1 (2000 Å) were prepared. The fabrication of device A (0.3 wt % of1b) follows:

EXAMPLE 5

[0040] The device A was assembled as follows: indium tin oxide (ITO)electrode with sheet resistance of 20 Ω/square on glass substrate, ahole transport material NPB (α-naphthylphenylbiphenyl amine) withthickness of 500 Å, an emitting layer made of mixture of 0.3 wt. %complex 1b and blue luminescent material Bepp₂ (berylliumbis(2-(2′-hydroxyphenyl)pyridine) with 400 Å thickness, an enhancedcharge transport layer LiF with thickness of 15 Å, and aluminum layerwith 2000 Å thickness. The metal and organic layers were laminated insequence under 5×10⁻⁶ mbar without breaking vacuum between differentvacuum deposition processes. The layers were deposited at rates of 2 or5 Å per second. The emissive area of the device as defined byoverlapping area of cathode and anode was 3×3 mm². The ITO coated glassslides were cleaned with organic solvents(acetone-isopropanol-methanol), deionized water, followed byultra-violet-ozone cleaner. EL spectra and currentdensity-voltage-luminance characteristics of the devices were measuredwith a spectrophotometer and a computer-controlled direct-current powersupply respectively at room temperature.

[0041] For these examples, the device external efficiencies increasewhen the doping concentration levels of complex 1b were adjusted from 2to 0.3 wt %. The specific examples are further illustrated as follows:

EXAMPLE 6

[0042] The performances of device A with 0.3 wt % doping level ofcomplex 1b are shown in FIG. 6. Two intense EL emissions at 453 and 540nm are observed when the device was driven under forward bias. Thecurrent density-voltage-luminance characteristics curves of device A arealso shown. The turn-on voltage is approximately 6-7 V. The maximumefficiency of the device was 4.1 cd/A at luminance of 2849 cd/m². Themaximum luminance of 9325 cd/M² was obtained at driving voltage of 10 V.The EL color of device A is yellow (CIE coordinates: x=0.33, y=0.47).

EXAMPLE 7

[0043] The performances of device B with 1.0 wt % doping level ofcomplex 1b are shown in FIG. 7. The device exhibits an intense ELemission peak at 546 nm and a weak emission at 457 nm. The onset voltageof device B was approximately at 6-7 V. The efficiency and maximumluminance were 1.9 cd/A at luminance of 1927 cd/m² and 6563 cd/M² atdriving voltage of 9.5 V respectively. The EL color of device B isyellow (CIE coordinates: x=0.39, y=0.54).

EXAMPLE 8

[0044] Device C with 2.0 wt % doping level of complex 1b exhibits anintense EL emission peak at 548 nm with an extremely weak emission ataround 450 nm (FIG. 8); EL efficiency of 1.5 cd/A was detected.Luminance of 6450 cd/m² was observed at driving voltage of 12 V. The ELcolor of device C is yellow (CIE coordinates: x=0.42, y=0.56).

[0045] Typically, doping level of greater than 5% is reported to achievedopant emission in organic or polymeric light-emitting devices. In thisinvention, the OLEDs show virtually complete emission of complex 1b whenthe doping level is around 2% and the efficiencies of devices increasefrom 1.5 to 4.1 cd/A when the doping levels of complex are decreasedfrom 2.0 to 0.3 wt %.

What is claimed:
 1. In an organic light-emitting device containing aheterostructure for producing electroluminescence, an emissive layer,comprising at least a host material and an emissive molecule, present asa dopant in said host material, wherein the said emissive molecule isselected from metal complexes bearing a quadridentate ligand containingat least one pyridine or substituted pyridine group.
 2. The emissivelayer of claim 1, wherein said emissive molecule is selected from metalcomplexes bearing a quadridentate ONNO-type ligand, where NN is2,2′-bypyridine or substituted 2,2′-bypyridine or 1,10-phenanthroline orsubstituted 1,10-phenanthroline.
 3. The emissive layer of claim 1,wherein the metal of said metal complexes is selected from Group
 10. 4.The emissive layer of claim 1, wherein said emissive molecule can have achemical structure represented by Formula I and II:

wherein M represents a Group 10 metal (including platinum) and R₁-R₁₄are each independently selected from the group consisting of hydrogen;halogen; alkyl; substituted alkyl; aryl; substituted aryl, withsubstitutents selected from the group consisting of halogen, lower alkyland recognized donor and acceptor groups.
 5. The emissive layer of claim4, wherein said emissive molecule is presented as a dopant in said hostmaterial in low concentration, including 0.3 to 2.0 weight % based onweight of host material.
 6. The emissive layer of claim 4 that produceselectroluminescence of yellow color.
 7. A method for turning color (CIEcoordinates) emitted by an organic light-emitting device comprisingemissive layer of claim
 5. 8. The emissive layer of claim 4, whereinsaid the host material is beryllium bis(2-(2′-hydroxyphenyl)pyridine(Bepp₂).
 9. The emissive layer of claim 4, wherein the said hostmaterial and said emissive molecule can be deposited as a thin layer bymethod of sublimation or vacuum deposition or vapor deposition orspin-coating or other methods.
 10. The emissive molecule is representedby Formula I in accordance with claim 4, wherein the R₁-R₅, R₇-R₈ andR₁₀-R₁₄ groups are proton atoms, R₆ and R₉ groups are tert-butyl groups,and M is platinum, namely:


11. The emissive molecule is represented by Formula II in accordancewith claim 4, wherein the R₁-R₅, R₇-R₈ and R₁₀-R¹⁴ groups are protonatoms, R₆ and R₉ groups are phenyl groups, and M is platinum, namely:


12. A method for the preparation of a light-emitting material having astructure represented by Formula I:

wherein M represents a Group 10 metal (including platinum) and R₁-R₁₄are each independently selected from the group consisting of hydrogen;halogen; alkyl; substituted alkyl; aryl; substituted aryl, withsubstitutents selected from the group consisting of halogen, lower alkyland recognized donor and acceptor groups.
 13. The method in accordancewith claim 12 wherein the R₁-R₅, R₇-R₈ and R₁₀-R₁₄ groups are protonatoms, R₆ and R₉ groups are tert-butyl groups, and M is platinum,namely:


14. A method for the preparation of a light-emitting material having astructure represented by Formula II:

wherein M represents a Group 10 metal (including platinum) and R₁-R₁₄are each independently selected from the group consisting of hydrogen;halogen; alkyl; substituted alkyl; aryl; substituted aryl, withsubstitutents selected from the group consisting of halogen, lower alkyland recognized donor and acceptor groups.
 15. The method in accordancewith claim 14 wherein the R₁-R₅, R₇-R₈ and R₁₀-R₁₄ groups are protonatoms, R₆ and R₉ groups are phenyl groups, and M is platinum, namely: